As this kind of the motor drive vacuum pump, there has been known one as shown in FIG. 1 of the accompanying drawing. In FIG. 1, the reference numeral 1 designates a motor which comprises a stator 1a consisting of magnetic poles each of which holds a field winding, a rotor 1b rotatably supported with respect to the stator 1a leaving a small air gap 1c therebetween, and a housing 1d holding the stator 1a and the rotor 1b therein. A vacuum pump 3 is attached to the motor 1 and is driven by a motor shaft 2. The references 4 and 5 respectively designate an intake port and a discharge port connected to the vacuum pump 3. The intake port 4 is connected to the vacuum tank 6. The reference 7 designates a check valve connected between the intake port 4 and the vacuum tank 6. The numeral 8 designates a drain hole formed in the housing 1d of the motor 1.
The operation of the device described above will be explained.
Actuation of the motor 1 causes revolution of the shaft 2 to drive the vacuum pump 3 with the result that air in the vacuum tank 6 is sucked through the check valve 7 and the intake port 4 and is sent pressurized to the discharge port 5 to be discharged in the atmosphere. The check 7 valve is to prevent air from reversely flowing into the vacuum tank 6 when the vacuum pump 3 is stopped, which keeps the vacuum tank 6 in an evacuated state.
In the conventional device having the construction described above, there has been a problem of noise generated from the discharge port 5. Especially, in a vane type vacuum pump which is so constructed that air sucked from the intake port 4 is compressed on the way to the discharge port 5 before discharged, the compressed air causes a great noise when it is discharged from the discharge port 5. Further, when vacuum (mmHg) in the vacuum tank 6 increases and there is a small amount of air to be sucked, there disadvantageously occurs noise due to sliding movement of the vanes.